A fuel injection amount of an internal combustion engine is set based on an intake air amount. However, during the so-called valve overlap period in which both of an intake valve and an exhaust valve are open, a part of the intake air which has passed through an intake passage may pass through a cylinder, and flows out to an exhaust passage. Thus, when the fuel injection amount is set based only on the intake air amount passing through the intake passage, the air/fuel ratio in the cylinder shifts toward a rich side by an amount corresponding to a scavenging amount, which is an amount of the air flowing out to the exhaust passage. As a result, the accuracy of air/fuel ratio control and the like decreases.
Therefore, it is necessary to estimate the scavenging amount, thereby setting a fuel injection amount based on the air amount actually existing in the cylinder. As a technology for estimating the scavenging amount, a method involving using an engine rotation speed, a valve overlap amount, and a supercharged pressure is disclosed in JP 2006-299992 A.